1. Field of the Invention.
The present invention relates to a hydrogen/oxygen fuel cell including a proton-conducting electrolyte which is solid at room temperature and onto opposing sides of which are applied a hydrogen electrode, which is negatively charged in use, is hydrogen permeable, and is in communication with a hydrogen-containing gas chamber, and an oxygen electrode, which is positively charged in use, is hydrogen permeable, and is in communication with an oxygen-containing gas chamber.
2. Background of the Related Art.
Such hydrogen/oxygen fuel cells are described in an article entitled "Assessment of Research Needs for Advanced Fuel Cells," published in an international journal ENERGY, Penner, S.S., (Ed.), Volume 11, No. 1/2, January/February, 1986, pages 137-152 (Special Issue).
This publication indicates that organic polytetrafluoroethylenes, for example, the product known under the trademark Nafion.RTM., have been employed as proton-conducting, solid electrolytes. The drawback of such solid electrolytes, however, is inherent in the materials themselves and is the fact that fuel cell operating temperatures must be limited to values below 100.degree. C. Even if the fuel gases are supplied under increased pressure, for example, 7 bar, the operating temperature can be raised only slightly above 100.degree. C. Although a solid electrolyte should be able to withstand a brief temperature increase to 200.degree. C., there exists, in principle, the danger at these temperatures of an irreversible change in the organic polyterafluoroethylene materials.
For future fuel cells, inorganic heteropolyacids of molybdenum and tungsten have been proposed as solid electrolytes. Heteropolyacids, however, have a defined water of crystallization content and, at higher temperatures, the water of crystallization may easily be driven off and change the physical characteristics of the material. Moreover, such solid electrolytes require uniform wetting since, otherwise, non-uniform current loads and temperatures occur which lead to cracks and so-called "hot spots". "Hot spot" as used herein refers to a location in an electrolyte at which the reaction gases have direct access to one another and react chemically with one another while developing heat exclusively.
According to the aforementioned ENERGY article, sintered nickel and alloys of the noble metals platinum and palladium have been employed as electrode materials. In addition to pure oxygen and hydrogen, air and forming gas have been employed as fuel gases.
Volume I entitled "Brennstoffzelle" ["Fuel Cell"] of the book dtv-Lexikon der Physik [Pocket Encyclopedia of Physics], published by Deutscher Taschenbuchverlag (1970), pages 300-307, and a publication by Rudolf Weber entitled "Der sauberste Brennstoff" ["The Cleanest Fuel"], published by Olynthus Verla fur verstandliche Wissenschaft und Technik, Oberbdzberg, Switzerland (1988), page 74, indicate that porous electrodes having open pores of a diameter of about 1 .mu.m must be employed for fuel cells. However, such pores may easily be filled with water formed during the reaction of the hydrogen and oxygen fuel gases. According to the first-mentioned publication, therefore, the porous electrodes are coated with a hydrophobic layer, such as a Nafion.RTM. or a polytetrafluoroethylene layer. This material, as discussed in the foregoing, has inherent temperature limitations.
Porous electrodes have the additional disadvantage that separation of hydrogen-containing gases and oxygen-containing gases from one another may be endangered if the electrolyte ceases to be a physical barrier, for example, due to the formation of cracks.
A basic problem in prior art fuel cells operating at temperatures around 100 .degree. C. is that the current intensity of the fuel cells is limited, even though a relatively high voltage is realized at these comparatively low temperatures for thermodynamic reasons. Current intensity is a function of hydrogen permeation and, as the operation temperature increases, the hydrogen permeation increases.
The operating temperature, moreover, should lie above the boiling point of water at the respective operating pressure so that the water formed evaporates on the oxygen side and is not deposited as a thin film on the oxygen electrode. Such a film would drastically reduce the performance of the fuel cell.
However, operating pressures above atmospheric pressure are preferred due to the higher conversion realizable. Therefore, it would be desirable to operate fuel cells at correspondingly high temperatures and particularly suitable materials for a solid electrolyte would be those substances which permit operation of a fuel cell under optimized thermodynamic conditions and, thus, with optimally matched thermodynamic parameters. The significant parameters which must be matched to one another are hydrogen permeation, operating temperature and operating pressure.
At the same time, fuel cells should be designed to avoid formation of hot spots to the greatest extent possible.